1. Field of the Invention
The present invention generally relates to a motor-driven power steering system for a motor vehicle or automobile which is adapted to assist a driver in manipulating a steering wheel or handle by making use of an assist torque generated by a reversible motor which is operatively coupled to the steering system. In particular, the invention is concerned with a control apparatus for the motor-driven power steering system which allows smooth transition of steering operation from a motor-aided mode to a manual mode when the assist torque generating motor falls into a locked or nearly locked state for some reason. More particularly, the invention is concerned with an apparatus for controlling an electromagnetic clutch through which an assist torque generated by the motor is transmitted to the steering system such that a coupling effort of the clutch is decreased continuously to a level allowing manual operation of the steering wheel when the motor falls into a locked or nearly locked state. Hereinafter, the combination of the motor-driven power steering system and the control apparatus will also be referred to as the motor-driven power steering control system.
2. Description of the Related Art
In the motor-driven (or electrically driven) power steering control system for a motor vehicle, a steering torque applied to a steering wheel by a driver is detected by a torque sensor to thereby allow a reversible electric motor of the power steering system to generate an assist torque of a magnitude which is substantially proportional to the detected steering torque while taking into account the speed of the motor vehicle (hereinafter referred to as the vehicle speed), wherein the assist torque thus generated is applied to a steering shaft assembly for aiding or facilitating the driver in manipulating the steering wheel.
In the power steering system of the type mentioned above, when the motor is locked due to some fault, the assist torque can no longer be generated. Besides, the locked state of the motor presents a great obstacle to the manipulation of the steering wheel, making it difficult or impossible to drive the motor vehicle comfortably.
As a motor-driven power steering control system which is equipped with measures for coping with the unwanted situation mentioned above, there is known a system disclosed in, for example, Japanese Unexamined Patent Application Publication No. 285460/1989 (JP-A-H1-285460).
FIG. 9 is a block diagram showing a structure of the motor-driven power steering control system described in JP-A-H1-285460. In this known motor-driven power steering system, a steering torque applied to a steering wheel or handle of a motor vehicle by a driver is detected by a torque sensor 101 in the state where an assist torque generating motor 100 is operatively connected to a steering shaft (not shown) by means of a clutch (also not shown). The detected torque signal outputted from the torque sensor 101 is applied to a torque neutral point detection circuit 102 via a differentiating circuit 116. In the torque neutral point detection circuit 102, a torque neutral point indicating a neutral position or state of the steering wheel is detected. The output of the torque neutral point detection circuit 102 is logically ORed with an output of a second level setting/comparison device 103 (at 120). A logical sum signal resulting from the ORing operation is supplied to a relay control circuit 117 via a self-hold circuit 118 for controlling a relay 105 interposed between a clutch driver 112 and a power source 111 such as an onboard battery. Simultaneously, the output of the torque neutral point detection circuit 102 is logically ORed with the output of a first level setting device 104 (at 121), the resulting logical sum signal being supplied to a clutch current control circuit 106.
With the structure described above, when it is detected by the torque neutral point detection circuit 102 that the magnitude of torque as detected by the torque sensor 101 is outside of a neutral region and when the current flowing through the motor 100 as detected by a motor current detecting circuit 110 exceeds a first level set at the first level setting device 104, a control signal is applied to the clutch current control circuit 106 which responds thereto by decreasing a clutch current supplied to a clutch driver 112. As a result of this, a slip takes place in an electromagnetic clutch 113 which serves to transmit a torque generated by the motor 100 to the steering shaft. In this manner, when the motor current increases due to semi-locking of the motor 100, the clutch current is decreased to loosen the mechanical coupling between the motor 100 and the steering shaft to thereby allow the steering wheel to be manipulated in a region close to the state where the steering wheel is manipulated without the aid of the assist torque. On the other hand, when the motor 100 is actually locked, accompanied with a further increase in the motor current, the relay 105 is opened to deenergize the clutch driver 112 to thereby set the manipulation of the steering wheel to the manual manipulation mode in the intrinsic sense. In this way, the steering wheel is protected against falling into the locked state or manipulation-heavy state even when the motor 100 is locked or nearly locked, whereby safety is assured for driving the motor vehicle. Parenthetically, the assist torque generating motor 100 is driven through a PWM switching driver circuit 114 under the control of a microcomputer 115 which is in charge of controlling the whole system.
The motor-driven power steering system described above however suffers from a problem that because the clutch is controlled on the basis of the output signal of the torque neutral point detection circuit 102 and the motor current detected by the motor current detecting circuit 110, it is impossible to decide instantaneously and accurately an occurrence of locking in the motor 100 as well as releasing thereof from the locked state. As a consequence, the clutch tends to be unnecessarily left in the slip state for a long time after the restoration of the motor from the locked state, whereby wear or deterioration of the clutch is accelerated, resulting in that the assist torque generated by the motor 100 can not effectively be transmitted to the steering shaft, thus presenting a problem.
As a motor-driven power steering system designed to cope with the problem mentioned above, there is known a motor-driven power steering control system as shown in FIG. 10.
Referring to FIG. 10, this known motor-driven power steering control system includes a torque sensor 111, a vehicle speed sensor 112, a current calculating/driving means 110 for arithmetically determining or calculating a motor current command value I.sub.O on the basis of a steering torque T derived from the output of the torque sensor 111 and a vehicle speed V derived from the vehicle speed sensor 112 in accordance with such relation as illustrated in FIG. 11, an assist torque generating motor 113 driven in accordance with the command value I.sub.O, and a clutch control means 120 for changing a control signal C for an electromagnetic clutch 121 from a high level (ON-level) to a low level (OFF-level) when the vehicle speed V derived from the vehicle speed sensor 112 has attained a reference vehicle speed V.sub.0, as illustrated in FIG. 12. A reference numeral 114 designates a motor current detecting means.
With the arrangement of the motor-driven power steering system described above, the clutch control means 120 sets the clutch control signal C at the high level so long as the vehicle speed V does not exceeds the reference vehicle speed V.sub.O to thereby allow the electromagnetic clutch 121 to be coupled to the motor 113, while the current calculating/driving means 110 controls the current supply to the motor 113 in accordance with the motor current command value I.sub.O determined on the basis of the vehicle speed V and the steering torque T, to thereby allow the motor 113 to generate an assist torque of corresponding magnitude.
On the other hand, when the motor vehicle is running at a high speed with the vehicle speed V exceeding the reference vehicle speed V.sub.0, the clutch control means 120 sets the clutch control signal C at the low level to thereby leave the electromagnetic clutch 121 disconnected from the motor 113, while the motor current command value I.sub.O generated by the current calculating/driving means 110 is reset to zero. In other words, so long as the motor vehicle is running at a speed higher than the reference level V0, the electromagnetic clutch 121 is disconnected to thereby exclude the possibility of the steering wheel falling into the locked or manipulation-heavy state. In this way, safety is assured in driving the motor vehicle.
With the motor-driven power steering control system described above, the electromagnetic clutch 121 can be coupled to or disconnected from the motor 113 instantaneously and accurately. However, the automatic operation of the electromagnetic clutch 121 in response to a change of the vehicle speed V around the reference vehicle speed V.sub.O is accompanied with harsh noise, which is far beyond acceptance from the environmental standpoint. Besides, shock is generated upon operation of the electromagnetic clutch 121 and transmitted to the driver, to his or her discomfort.
As a technique which tackles the solution of the above problem, there may be mentioned a motor-driven power steering control system which is disclosed in Japanese Unexamined Patent Application Publication No. 85391/1993 (JP-A-HS-85391).
FIG. 13 is a block diagram showing this known motor-driven power steering control system. In this figure, components same as or equivalent to those shown in FIG. 10 are denoted by like reference symbols. Referring to FIG. 13, the motor-driven power steering control system is similar to that shown in FIG. 10 in that it includes a torque sensor 111, a vehicle speed sensor 112, a current calculation/driver means 110 for arithmetically determining or calculating a motor current command value I.sub.O on the basis of a steering torque T derived from the output of the torque sensor 111 and a vehicle speed V derived from the output of the vehicle speed sensor 112, and an electromagnetic clutch 121, and differs from the latter in that there are provided a clutch current detecting means 130 and a clutch control means 131 which is adapted to operate in cooperation with the clutch current detecting means 130.
The clutch current detecting means 130 detects a clutch current I.sub.C which actually flows through the electromagnetic clutch 121 on the basis of the clutch control signal C supplied from the clutch control means 131. To say in another way, the clutch control means 131 generates the clutch control signal C in order to set the clutch current I.sub.C for the electromagnetic clutch 121.
More specifically, so long as the vehicle speed V is lower than the reference vehicle speed V.sub.O, the clutch current detecting means 130 outputs the clutch current I.sub.C of a level I.sub.V to drive the electromagnetic clutch 121 for allowing the motor 113 to be linked to the steering shaft so that an assist torque of a magnitude which decreases linearly as the vehicle speed increases is available, as is illustrated in FIG. 14. On the other hand, when the vehicle speed V is higher than the reference vehicle speed V.sub.O (e.g. 50 km/h) inclusive, the clutch current detecting means 130 outputs the clutch current I.sub.C of a constant level I.sub.CR which is lower than the level I.sub.V, as a result of which the coupling power of the electromagnetic clutch 121 is enfeebled.
In this way, the assist torque generated by the motor 113 is controlled in accordance with the vehicle speed V without disconnecting completely the coupling established between the steering shaft and the assist torque generating motor via the electromagnetic clutch 121, whereby generation of noise and shock upon coupling/disconnecting operation of the electromagnetic clutch 121, as experienced in the case of the system shown in FIG. 10, is effectively prevented.
The motor-driven power steering system disclosed in JP-A-H5-85391 however suffers a problem that the manufacturing cost therefor becomes very high because of the necessity for the clutch current detecting means 130 implemented in a complicated circuit configuration for executing intricate processing in order to detect and control the clutch current I.sub.C. Besides, control of the clutch current I.sub.C requires processing of high accuracy which is difficult to perform in practical applications.